Electric discharge lamp containing metallic iodides including sodium iodide



6, 19% J. VILLAIN' ET AL 3,488,540

ELECTRIC DISCHARGE LAMP CONTAININGMETALLIC IQDIDES INCLUDING SODIUMIODIDE Filed March 13, 1967 9 Invervtors A 7 Jacques VILLam a PhiLippeWague t Pa triqk erbet Then A t irorneg United States Patent O Int. Cl.H01j 1/02, 7/24 US. Cl. 313-25 6 Claims ABSTRACT OF THE DISCLOSURE Theuseful life of metallic iodide lamps has been relatively short bycomparison with that of ordinary mercury vapor lamps. Towards the end oflife, luminous efficiency decreases quite rapidly, and the useful lifeis ended when an inordinate increase in starting and restarting voltagetakes place. These disadvantages are related to the diffusion of sodiumthrough the wall of the quartz arc tube and, as a consequence, theappearance of free iodine within the arc tube. The occurrence of thesephenomena is prevented by the invention which is characterized in onemode by the introduction into the arc tube of a quantity of silicon,suitably as crushed crystals, and in another mode by the incorporationof silicon in fine powder form into the thoria serving aselectron-emissive material on the cathodes, the silicon preventing themigration of sodium through the quartz wall.

BACKGROUND OF THE INVENTION The present invention relates to highpressure mercury vapor lamps in which metals having low ionizationpotentials are added in the form of volatile halides in order to obtainthe emission of white light; the metals thus introduced in definiteproportions are thallium, indium, and sodium.

These lamps comprise a burner or are tube of silica located within anouter protective jacket of hard glass, generally evacuated, andelectrodes each constituted by a tungsten shank on which are wound twosuperposed layers of tungsten wire, the space between turns being filledwith thoria whereof the reduction entails the formation of an emissivefilm of thorium at the end of the tungsten shank, the equilibriumbetween the speed with which thorium disappears from the film and thespeed with which thorium is produced by reduction depending upon thetemperature of the electrode.

Metallic halide lamps have lagged in their commercial development byreason of their relatively short useful lives by comparison with thoseof ordinary mercury vapor lamps, their utility being limited by anincrease in starting and restarting voltage and by a decrease inluminous eificiency.

In the state of the art prior to our invention, even for lamps utilizingiodides of great purity and containing no trace of humidtiy, the usefullife was between 1000 and 2000 hours, the drop in luminous efficiencybeing as much as 30 to 50%.

Experience has shown that even though very high at the first lighting,the starting voltage drops considerably after a few tens of hours ofoperation, then climbs again and exceeds its initial value after a fewhundred hours, so that the lamp becomes unutilizable because in practiceit can no longer be started.

As the starting voltage increases, the extremities of the arc tubebecome coated with a deposit of tungsten which extends progressivelyover the entire wall and with a localized spot of thorium iodide whichdevelops until finally a sudden increase in the restarting voltageprovokes the extinction of the lamp.

ANALYSIS OF PROBLEM It has appeared to us that these phenomena arelinked to the presence of sodium iodide and that they occur according toa mechanism which will now be explained; it has also appeared to us thatit is possible to avoid these phenomena and, by so doing, achieve lampshaving long lives even though containing sodium iodide.

In the vicinity of the hottest parts of the arc tube wall, therecombination of atoms of iodine and sodium is not total; the sodium notrecombined causes a progressive attack of the silica which manifestsitself by an alteration of its transparency and a gray veiling. Thisattack, localized in the hottest regions, results in the formation ofsodium silicate and silicate monoxide; so long as the attack is weak,the strength of the arc tube is not diminished.

When the attack has propagated itself throughout the thickness of thequartz, the sodium diflfuses through the wall and condenses on thecolder portions of the outer protective jacket forming a bright metalliccoating, while the arc tube fills with iodine which appears in the formof mercuric iodide upon cooling.

The iodine liberated in the arc tube or burner is by reason of itselectronegativity the cause of the augmentation of the starting andrestarting voltage; it has for effect to destroy the film of thorium onthe electrode ends thereby provoking the apeparance of thorium iodidespots. Since the equilibrium betweeen the formation and the destructionof the emissive film can only maintain itself by a rise in temperatureand an increase in cathode fall, the electrodes sputter tungsten whichdeposits on the wall. When the reserve of thorium is entirely exhausted,a sudden increase in the restart voltage brings about the extinc tion ofthe lamp.

It has been observed that the migration of sodium through the quartzoccurs more rapidly inasmuch as the temperature of the arc tube wall ishigher; when blackening begins to occur, the migration of sodiumaccelerates for the reason that the temperature of the wall increases asit darkens.

From the foregoing observation, it follows that any cause direct orindirect of a rise in temperature of the arc tube wall has for effect toaccellerate the migration of sodium and, in consequence, the liberationof iodine.

For equal power input, a reduction in the size of the arc tubeeaccelerates the dilfusion of sodium on account of the direct increasein temperature of the wall.

The use of insufiiciently emissive electrodes accelerates diffusion ofsodium by reason of the blackening provoked by pulverization orsputtering; similarly, the introduction of iodine in the burneraccelerates the diffusion of sodium on account of the blackening whichresults from the attack of the electrodes.

The cumulative action of these causes and effects explains the rapidtermination of life of iodide lamps as soon as a reduction in thetransparency of the arc tube wall occurs.

Means which purpose to increase luminous efliciency by reducing heatlosses, such as for instance a higher vacuum in the outer jacket, heatinsulation of the ends of the arc tube, or a layer on the outer jacketwhich reflects infrared rays back onto the arc tube, lead to a reductionin life from the fact that these means increase directly the temperatureof the wall; in the same vein, it has been observed that an iodide lampdarkens more rapidly in a fixture than in free air on account of theadditional heating of the outer jacket.

The introduction of free iodine into the arc tube which has beensuggested for avoiding attack of the wall by sodium, produces an effectopposite to that hoped for,

the free iodine accelerating the diflusion of sodium by reason of theblackening due to attack of the electrodes; besides the increase instarting voltage and absorption of light by the Wall, the luminousefiicacy is reduced by reason of the more rapid recombination of theiodides.

The introduction or the appearance of oxygen in the arc tube facilitatesthe attack of silica by sodium and for this reason should be avoided.

In an iodide lamp emitting white light, the migration of sodium causes achange in the spectral composition while the liberation of iodineentails a lowering in luminous efiicacy.

The present invention has for object to suppress the migration of sodiumthrough the quartz wall and, as a result, the appearance of free iodinein the arc tube or burner. There will thus be obtained a considerableincrease in the useful life of iodide lamps by maintaining throughthousands of hours of service, the constancy of the starting voltage, ofthe restarting voltage, of the ef fective operating voltage, of theelectric power absorbed, of the luminous efficacy, and of the spectralcomposition of the light.

SUMMARY OF INVENTION The present invention is characterized by theintroduction into the arc tube of an appropriate quantity of silicon,suitably as crushed crystals, or by the incorporation into the thoria ofthe electrodes of an appropriate quantity of silicon, suitably as a finepowder. These two modes of realization of the invention may be usedseparately or simultaneously.

Conformably to the principles which have led to the elaboration of thepresent invention, it is important to eliminate in the first place allcauses of spots and darkening.

Following a complementary characteristic of the invention, theelectrodes should present a large emissivity and the iodides shouldcorrespond to the smallest valency of the metals; there must not be anyaddition of free iodine in any appreciable quantity, but the provisionof a controlled small quantity of iodine may facilitate the formation ofthe lamp when the silicon is introduced into the arc tube in the form ofcrystals.

The two modes of realization of the invention lead to the same resultsbut by dilterent processes; in the first mode of realization, theincrease in emissivity is due primarily to the condensation on theelectrodes of a part of the silicon introduced into the arc tube, whilein the second mode of realization, the protection of the wall comesprimarily from the vaporization of a part of the silicon incorporatedinto the thoria.

BRIEF DESCRIPTION OF THE DRAWING In the drawing:

FIG. 1 shows in side view a mercury sodium iodide arc lamp in which theinvention is embodied;

FIG. 2 is an enlarged fragmentary cross section of one of theelectrodes.

FIG. 1 of the drawing illustrates a high pressure metallic iodide lamp 1comprising a quartz arc tube 2 enclosed within a vitreous jacket 3,provided with a screw base 4. The are tube contains an inert startinggas, suitably argon at a low pressure, for instance 25 millimeters ofmercury; a quantity of mercury which is entirely vaporized duringoperation of the lamp with the generation of a pressure in the range of1 to atmospheres; and a quantity of sodium iodide which is in excess ofthat vaporized at the operating temperature which should be not lessthan 500 C. at any place. Lesser quantities of thallous iodide may alsobe provided in order to improve the spectral quality of the light.

The main electrodes 5, 6 at opposite ends of the arc tube comprise apair of generally concentric coils or helices of tungsten wire 7, 8, theformer being tightly wound around the tungsten shank or inlead 9 and thelatter being screwed on the former as shown in FIG. 2. The inside helix7 has a greater Winding pitch in its middle portion whereby to leavecavities which are filed with an electron-emitting mixture 10 consistingprimarily of thoria. An auxiliary starting electrode 11 close to mainelectrode 5 permits starting at a lower voltage. In order to reduce heatlosses, the interenvelope space is evacuated prior to scaling off theouter jacket. Also a heat-reflective coating, indicated by speckling 12,is applied to the ends of the arc tubeand the adjacent portions of theseals.

According to the first mode of realization of the invention, a quantityof silicon realtively small by comparison with the sodium iodide isintroduced into the arc tube in the form of crushed crystals shown at 13in FIG. 1 for the purpose of forming a small quantity of silicon iodideand of depositing a film of silicon on the wall.

The provision of a quantity of iodine relatively small by comparisonwith that of silicon, preferably in the form of mercury iodide, has theadvantage of reducing the time of formation of the lamp and of notborrowing iodine from the iodides of thallium and indium, so that itdoes not entail a possible change in the spectral com position of thelight during the initial period of operation of the lamp.

As soon as the silicon iodide has formed and the film of silicon hasbeen deposited, an equilibrium is established on which depends themaintenance of the electrical and photometric characteristics of thelamp.

The iodide of silicon facilitates the recombination of iodine and sodiumin the vicinity of the Wall, the sodium decomposing the silicon iodideto form sodium iodide and liberate silicon which recombines with theiodine; in addition, the film of silicon constitutes an elfectiveprotection of the silica or quartz Wall against the aggressive action ofthe sodium, the formation of silicide of sodium not being possible atthe temperature of the wall.

Migration of sodium and liberation of iodine being thus avoided, therelative proportions of the diiferent iodides do not change with timeand none of the physical or chemical effetcs due to theelectro-negativity of the iodine can produce themselves; it follows thatthe starting voltage at each lighting of the lamp, the restartingvoltage at each half-cycle of current, the efiFective operating voltageand the electric power absorbed are maintained constant, as also theluminous efficacy and the spectral composition of the light.

The emissivity of the electrodes is considerably aifected by theslightest destruction of the thorium film.

The equilibrium between the speeds of disappearance and of production ofthorium is accompanied by a reduction in cathode fall and a decrease intemperature of as much as 300 at the ends of the electrodes, whencefollows important reduction in pulverization and vaporization.

The losses at the electrodes being smaller, the luminous efiicacy isincreased; the consumption of the reserve of thoria being less rapid,the life of the electrodes is con siderably augmented.

The silicon may be placed at any place on the wall so that the procedureis useful no matter what the operating position of the lamp; it isnonetheless important to avoid having silicon in contact with a portionof the electrodes at a temperature exceeding 2000 C. in order to avoidthe production of fusible tungsten silicide.

The formation of the lamp is accompanied by the appearance of a smallspot of thorium silicide in the colder region of the wall; this smallspot is characteristic of the use of this procedure.

According to the second mode of realization of the invention, anappropriate quantity of silicon is added to the thoria 10 of theelectrodes in the form of a fin powder in order to avoid reduction and,as a consequence increase emissivity.

As before, increase in emissivity is evidenced by a reduction ofapproximately 300 in the temperature of the extremities of theelectrodes.

By reason of the increase in emissivity and reduction in temperature, nodarkening tends to be produced on the burner wall; the reduction ofthoria by the silicon does not produce any liberation of oxygen whichcould facilitate the difiusion of sodium.

The addition of powdered silicon to the thoria presents, -by comparisonwith the introduction of crystals of silicon into the burner, theadvantage of avoiding the risk of formation of fusible tungstensilicide.

The two modes of realization of the invention can be used in combinationwith the structure which is the subject matter of the patent applicationof the assignee herein deposed in France 13th of January 1966 having fortitle Apparutus Applicable to Electric Discharge Lamps ContainingMetallic Iodides in Saturated Vapor Notably Sodium Iodide, suchapparatus having for effect to increase the luminous efiicacy of iodidelamps operating vertically without entailing an increase in temperatureof the burner wall.

Lamps of 400-watts input realizing the conditions of the inventionherein have operated 6000 hours with a luminous efficacy of 95 lumensper watt without modification of color appearing during that time.

By way of example, for a 400-watt lamp, the quantity of silicon utilizeddoes not exceed milligrams in the form of crystals introduced into thearc tube and 2 milligrams in the form of powder added to the thoria ofthe electrodes.

Since the invention permits an increase in temperature of the wall, itstwo modes of realization may be used in combination with known means forreducing heat losses, notably those consisting of providing a highvacuum in the outer jacket, providing a heat-reflective coating on theends of the arc tube, or coating the protective outer jacket with asubstance reflecting infrared rays back onto the burner.

If the crystals of silicon surround to a suitable depth the lowerelectrode of a vertically operating lamp, they produce the same effectof enhancement of the luminous emission of sodium as the apparatus whichis the subject matter of the patent application of the assigneepreviously cited.

The procedure which is the object of the present invention is applicableto lamps containing lithium iodide.

Obviously the two modes of realization of the present invention whichhave been described are not limitative; all analogous procedures basedon the same principles and leading to the same results, notably thoseconsisting in introducing into the burner a small quantity of siliconiodide or in utilizing an element other than silcon, would lkewise comewithin the scope of the present invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A high pressure are discharge lamp comprising a quartz envelopecontaining a filling of mercury, metal halide including sodium iodide,and inert gas at a low pressure, thermionic electrodes sealed into theends of the envelope comprising a tungsten structure supporting aquantity of thoria, the iodides within said envelope corresponding tothe smallest valencies. of their metals, and a small quantity of siliconenclosed within said envelope for protecting the quartz wall againstattack by sodium.

2. A lamp as in claim 1 wherein the metal halide filling includesbesides sodium iodide, thallium iodide and indium iodide.

3. A lamp as in claim 1 wherein the silicon is provided in the form ofcrystals not in contact with an electrode.

4. A lamp as in claim 1 wherein the silicon is provided in the form of afine powder admixed with the thoria of the electrodes.

5. A lamp as in claim 1 wherein the arc tube is enclosed in an evacuatedouter jacket.

6. A lamp as in claim 5 wherein the ends of the arc tube are providedwith a heat-reflecting coating.

References Cited UNITED STATES PATENTS 2,162,414 6/1939 Abbott 313-211 X2,251,046 7/1941 Gaides et al 313-211 3,219,868 11/1965 Mason et al.313-184 3,234,421 2/1966 Reiling m. 313-25 3,349,276 10/1967 Jacobs eta1. 313-211 3,350,598 10/1967 Corbin et a1. 313-220 FOREIGN PATENTS1,011,591 12/ 1965 Great Britain.

JAMES W. LAWRENCE, Primary Examiner P. C. DEMEO, Assistant Examiner US.Cl. X.R.

